Electrically connecting a device within a vacuum chamber to a power supply positioned outside the vacuum chamber typically requires an electrical feed-through—an airtight passage through a wall of the vacuum chamber in which passage one or more conductors are provided. A conductor of the feed-through is generally insulated from the wall of the chamber using an insulator, for example a ceramic material. Such electrical feed-throughs are commonly used to make an electrical connection from the exterior of the vacuum chamber to devices installed within the vacuum chambers. Typically electrical power has to be supplied to devices installed within the vacuum chambers such as thermal evaporation sources, substrate heaters etc, and/or electrical signals have to be retrieved from devices installed within the vacuum chambers such as temperature sensors, measurement devices, etc.
Many vacuum processes, e.g. physical vapour deposition, generate a substantial amount of heat, which raises the temperature inside the vacuum chamber to relatively high temperatures, i.e. above about 200° C. Over time elements originating from the vacuum process or residuals left in the vacuum chamber may degrade the insulating capacity of the electric feed-throughs by, for example, condensation of electrically conducting elements on the insulator of the feed-through resulting in an electrical short circuit between conductors extending through the feed-through or between the conductor and the vacuum flange. Moreover the exposure to reactive elements from the vacuum chamber may lead to chemical reactions or corrosion of the materials forming the joint between the conductor and the insulator, which eventually breaks the vacuum seal.
For example, Cu(InGa)Se2-based thin film solar cells (CIGS) are fabricated by high vacuum co-evaporation of the elements Cu, In, Ga and Se at a substrate temperature of about 500° C. Since most of the vacuum chamber interior is heated to temperatures above 200° C., and not all Se immediately reacts to form Cu(InGa)Se2, a partial pressure of Se in excess of 10−6 mbar is present in the chamber. Se will thus essentially come into contact with all the surfaces of the interior of the vacuum chamber. These surfaces will include the electrical power feed-throughs to the substrate heaters and the evaporation sources. In this situation Se may condense on the insulators of the feed-throughs, possibly also react chemically with other elements that have reached the same location, thereby forming an electrically conductive layer on the insulator of the feed-through resulting in short circuits and consequent malfunction of the substrate heaters and/or the evaporation sources. Se that reaches the electrical vacuum feed-through can also react chemically with the materials constituting the joint between the conductor and the insulator. This may result in corrosion and breakdown of the vacuum seal.
Short circuiting of the conductors of the electrical feed-through and breakdown of the vacuum seal is particularly a problem for production equipment, if one or more devices within the process chamber malfunctions due to the short circuit or the vacuum pressure begins to rise due to a leak. Such problems necessitate costly and time-consuming shut downs of the production equipment to replace the electrical feed-through. One example of this is an in-line production apparatus for Cu(InGa)Se2-based thin film solar cells (CIGS) where one or more evaporation sources may cease to operation, which has a detrimental effect on the thin film properties, or where there is a leak at the electrical feed-through which makes it impossible to reach the desired vacuum pressure.